In the refrigeration/air-conditioning field, 1,1,1,2-tetrafluoroethane (R134a), R410A as a mixed refrigerant of difluoromethane (R32) and pentafluoroethane (R125) in a mass ratio of 1/1, and the like, which are hydrofluorocarbons (HFC), are currently widely used as refrigerants for coolerators, car air-conditioners, room air-conditioners, industrial refrigerators, and the like.
These HFC refrigerants, however, have a high global warming potential (GWP) of 1000 or more while having an ozone depletion potential (ODP) of zero, and thus the use thereof comes to be limited by the so-called F-gas regulation for the purpose of global environment protection.
As alternatives of the refrigerant high in GWP, 2,3,3,3-tetrafluoropropene (HFO-1234yf) and difluoromethane (R32) itself have been studied as candidates in terms of their thermodynamic characteristics. In addition, a mixed refrigerant of such a refrigerant and other refrigerant, which is well-balanced with respect to GWP and various characteristics, has also been studied. An alternative of the HFC refrigerant is required to be low in GWP, and the GWP of HFO-1234yf is as low as 4. Although the GWP of R32 is as slightly high as 675, R32 has been studied as a major candidate because the gas pressure thereof is high and R32 is a highly-effective refrigerant (for example, Patent Literature 1).
In addition, hydrocarbon refrigerants such as isobutane (R600a) and propane (R290), which have been already in practical use for coolerators, have been studied as alternative refrigerants while they are flammable, because the GWP is as low as 20 or less and physical property values are suitable.
When a refrigerant whose pressure is high, such as R32 or a mixed refrigerant including R32, among alternative refrigerant candidates, is used, the discharge temperature in a compressor is higher, thus the oil film of a refrigerating machine oil for lubricating the inside of the compressor is thinner and lubricating conditions are severer, and therefore a refrigerating machine oil good in lubricity and stability is required.
In addition, in the case of the hydrocarbon refrigerant, not only fluorine that results in an increase in lubricity is not present in the hydrocarbon molecule and thus the refrigerant cannot be expected to exert the lubricity enhancement effect unlike the HFC refrigerant and the like, but also the solubility of the hydrocarbon refrigerant in the refrigerating machine oil is high to reduce the viscosity of the oil, thereby making lubricating conditions severer, and it is demanded for the refrigerating machine oil that its antiwear property is equal to or higher than that of a conventional one.